To improve the working environment by pouring concrete separately in several times for a vertical structural body, placing a deck plate on the ALC formwork when building a top portion and then building a horizontal structural portion on the deck plate, thereby releasing the ceiling and disassembling and taking out the formwork in a pit.

A vertical structural body 11 by mesh method or the like is built by dividing it to several portions, installing a beam formwork 17, placing an ALC formwork 18 on a flange 19 formed on the upper end of the beam formwork 17, and fixing it with bolts and nuts. The top portion 18a of the ALC formwork 18 is to be position to the height almost equal to the lower end surface of a horizontal structural body to be built next. When pouring concrete to the beam formwork 17 constituted as stated above, the ALC formwork 18 is to be buried in the vertical structural body 11. After curing and hardening of concrete, the beam formwork 17 can be disassembled and carried out in a state where the ceiling is released, both the working environment and working efficiency can be improved.